1. Field of the Invention
The present invention relates to an ink jet printing apparatus and a preliminary ink ejection method executed following a suction-based ink ejection performance recovery operation.
2. Description of the Related Art
Printing apparatus used as a means for printing images in printers, copying machines and facsimiles or printing apparatus used as output devices of composite electronic machines and workstations including computers and word processors are designed to print images on print media, such as paper and plastic thin plates, according to image information (all output information including character information). These printing apparatus can be classified into an ink jet system, a wire dot system, a thermal system and a laser beam system in terms of a printing method employed. The printing apparatus of ink jet system (hereinafter referred to as an ink jet printing apparatus) forms an image by ejecting ink from a printing means including a print head onto a print medium and has an advantage of being able to enhance a resolution more easily than other printing systems. Other advantages include a fast printing speed, low noise and low cost. As color outputs such as color images have an increasing importance in recent years, a growing number of color ink jet printing apparatus capable of producing a high image quality comparable to that of a silver salt picture are being developed.
In such ink jet printing apparatus, to improve a printing speed, a print head with an array of integrated printing elements generally has a plurality of ink ejection openings and liquid paths integrally formed therein. To deal with color printing, a printing apparatus with a plurality of print heads, one for each of different ink colors, has come into wide use.
FIG. 1 shows main components of the printing apparatus for printing on paper using a print head. In the figure, designated 101 are ink jet cartridges each of which includes an ink tank containing one of four color inks—black, cyan, magenta and yellow—and a print head 102 having a nozzle array assigned to that ink color.
FIG. 2 is a schematic diagram of the print head of FIG. 1 as seen from a direction z. A plurality of ejection openings (also referred to as “nozzles”) are arranged in columns by ink colors. Designated 201 are nozzles that are formed in the print head 102 at a density of D nozzles per inch (D dpi) and can eject 10 pl of yellow ink. Nozzles capable of ejecting 10 pl of ink are called “large nozzles” and dots formed by ink droplets ejected from the large nozzles are called “large dots.” Denoted 202 are nozzles smaller in diameter than the large nozzles and capable of ejecting 5 pl of yellow ink. The nozzles that eject 5 pl of ink are called “small nozzles” and dots formed by ink droplets ejected from the small nozzles are called “small dots.” Likewise, 203, 205 and 207 represent large nozzles for magenta, cyan and black inks, respectively, and 204, 206 and 208 represent small nozzles for magenta, cyan and black inks, respectively.
The large nozzles and small nozzles for each color ink are formed at front ends of liquid paths 210 extending from one and the same liquid chamber 209.
Returning back to FIG. 1, designated 103 is a paper feed roller 103 which rotates in a direction of the shown arrow together with an auxiliary roller 104 to hold a print medium P between them and feed it in a y direction (sub-scan direction). Denoted 105 are a pair of paper supply rollers to supply a print medium. The paired paper supply rollers 105 rotate holding the print medium P in between, as with the rollers 103 and 104, but their rotating speed is set smaller than that of the paper feed roller 103 to create tension in the print medium. Denoted 106 is a carriage which supports the four ink jet cartridges 101 and scans them as they eject ink. The carriage 106 is situated at a home position h indicated by a dashed line in the figure when no printing operation is performed or when the print head 102 is subjected to an ejection performance recovery operation by a suction device 107.
The recovery operation includes a suction-based recovery operation. This operation sucks out and discharges viscous ink, bubbles in the print head liquid chamber and mixed inks by the suction device 107 installed in the ink jet printing apparatus. The suction-based recovery operation normally involves capping a face of the print head, i.e., nozzle-formed surface, with a cap and then creating a negative pressure in the cap by a pump means such as a tube pump or piston pump. The negative pressure thus generated causes the ink in the print head liquid chamber to be sucked and discharged out of the print head through the print head nozzles. Immediately after the suction operation, however, the ink sucked out into the cap remains on the print head face and this residual ink may flow back into the print head. This reverse flow may result in the viscous ink remaining in the liquid chamber 209 of the print head. When the print heads of multiple colors are capped with a single cap for recovery operation, this reverse flow causes color ink mixing.
Therefore, after the suction-based recovery operation is executed, viscous ink and mixed inks are ejected out into the cap until these inks are completely discharged from the head. This recovery operation is called a preliminary ejection.
The amount of power supplied from a power source to drive the print head is set assuming a normal printing condition. So, if during the preliminary ejection operation all nozzles are activated simultaneously for ejection, the power consumption exceeds the amount of power supply. Thus, all the nozzles cannot be driven at the same time and normally the nozzles of the print head are divided into some groups that undergo the preliminary ejection operation at different times.
For example, after the suction-based recovery operation is done, the large nozzles each perform 20,000 preliminary ejections at a frequency of 10 kHz, followed by each small nozzle performing 20,000 preliminary ejections at a frequency of 10 kHz. This preliminary ejection cycle can discharge viscous ink and mixed inks. The preliminary ejection cycle that follows the suction-based recovery operation takes 4.0 seconds.
During the preliminary ejections, as during the ejections for normal printing, the ejected ink does not fly as a single droplet but is split into a plurality of ink droplets. A biggest ink droplet of these split droplets is called a main droplet, smaller ink droplets following the main droplet are called satellites, and finer droplets flying at slower speeds are called stray mist.
FIGS. 3A to 3C schematically illustrate how the main droplet, satellites and stray mist are formed at the time of ink ejection.
Denoted 301 is ink, 302 ink immediately after being ejected, 303 a meniscus, 304 a main droplet, 305 satellites and 306 stray mist.
An ink ejection initiates as shown in FIG. 3A. Immediately after the ejection, the ink 302 is shot continuously from a nozzle. Then, as shown in FIG. 3B, the meniscus 303, formed by the contraction of a bubble or the deformation of a piezoelectric element, retracts, causing the ink 301 to move into the interior of the print head 102. As the ink 301 moves inwardly, the projected ink 302 separates from the ink in the print head, with the result that a speed distribution is generated in the flying ink 302. As shown in FIG. 3C, the ink with a speed distribution is split into a droplet with the largest volume and the highest speed (main droplet 304), ink droplets with smaller volumes and slower speeds (satellites 305), and ink droplets with even smaller volumes and slower speeds (stray mist 306) that do not reach the interior of the cap.
The preliminary ejection is carried out in the cap of the suction device 107 so that most of the ejected ink is accommodated in the cap. However, the stray mist 306 with small volume and slow speed does not reach the cap but floats around the print head, adhering to the print head face. If, for example, the stray mist adheres to transport rollers or others, not only does it stain the transport rollers, but this stain is also transferred onto the print medium, degrading the image quality.
The volume of the stray mist 306 increases as the number of preliminary ejections and the ejection frequency increase and the volume of ink ejected from each nozzle decreases. When the number of preliminary ejections increases, the volume of stray mist 306 increases proportionally. In a high-frequency preliminary ejection operation, an air flow is generated among nearby nozzles by the ink droplets ejected at high frequencies and this air flow in turn swirls up mist which easily adheres to the print head face. The satellites 305 produced from nozzles of a large ejection volume have a sufficient mass and speed to land on the cap, whereas satellites 305 produced from nozzles of a small ejection volume have an insufficient mass and speed to reach the cap. The latter satellites therefore are likely to become stray mist 306. Such an increase in the stray mist 306 results in an increase in stain.
Therefore, the preliminary ejection operation performed after the suction-based recovery operation may take long depending on the number of preliminary ejections and the ejection frequency.
Further, depending on the ink volume ejected by the preliminary ejections, the number of preliminary ejections performed and the ejection frequency, most of the stain due to the stray mist adheres to the interior of the ink jet printing apparatus, from which the stain is further transferred onto a print medium, making it impossible to produce a desired image.
As described above, the conventional ink jet printing apparatus is required to execute preliminary ejections after the suction-based recovery operation, and the time taken by the preliminary ejection operation varies depending on the number of preliminary ejections and the ejection frequency. Thus, performing sufficient preliminary ejections on each nozzle will take some time. The combined execution of the suction-based recovery operation and the preliminary ejection operation therefore will take long, giving the user an impression of a long wait after a power-up of the apparatus before the printing actually starts.
Further, depending on the ink volume of preliminary ejections, the number of preliminary ejections performed and the ejection frequency, a large amount of stray mist may be produced and adhere to the print head face. This in turn may affect the direction of ink ejection during the printing operation or cause mixing of color inks. The stray mist may also adhere to transport rollers or other components in the printing apparatus, from which the ink mist may be transferred as stain onto the print medium, degrading a printed image quality.